<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html,body{
            height: 100%;
        }
        *{
            margin: 0;
            padding: 0;
        }
     .box{
        height: 100%;
        display: flex;
        flex-direction: column;
     }
     .box1{
        height: 60px;
        background-color: #373d41;
     }
     .box2{
        flex: 1;
        display:flex ;
     }
     .box2-a{
        width: 200px;
        height: 100%;
        background-color: #333744;
     }
     .box2-b{
        flex: 1;
     }
    </style>
</head>
<body>
    <div class="box">
        <div class="box1"></div>
        <div class="box2">
            <div class="box2-a"></div>
            <div class="box2-b"></div>
        </div>
    </div>
</body>
</html>